Gas-lifting is an artificial lifting method used to produce oil from wells that do not flow naturally. In gas-lifted wells, gas is injected through the well annulus and into the well tubing at a down-well location. The gas mixes with the oil in the tubing, aerating the oil and causing it to rise to the surface.
To pass through the annulus to the tubing, the injection gas flows through a valve commonly referred to as a gas lift valve. Gas lift valves are one-way valves that allow gas to pass from the annulus to the tubing but prevent oil from passing through to the annulus. Most valves contain a pressurized bellows and an internal check valve. The bellows opens when the injection gas is pressurized above a threshold value, and the internal check valve prevents oil from passing through the gas lift valve.
A gas lift valve can fail if it allows oil passage from the tubing to the annulus. For failure to occur, at least two conditions are simultaneously met: (1) the reverse-flow check valve has a leak and the tubing pressure exceeds the gas pressure; and (2) a combination of high tubing pressure and low gas pressure allows the bellows valve to open. Further, if both the check valve and the bellows valve leak, then backflow can occur any time the tubing pressure exceeds the annulus pressure.
Proper function of gas lift valves is very important for the safety of the well and surface operations. For example, if hydrocarbons flow through the annulus and reach the wellhead, an undesired accumulation of high-pressure combustible materials may occur at the well and surface, and hence may increase the risk of injury to personnel and/or damage to equipment, as well as disruptions in operation. Gas lift valves are thus susceptible to leakage, with seal corrosion being a major contributing factor.
Existing gas-lift safety valve designs are designed to actuate and close in the event of temperatures changes (signifying possible leakage of the valve), preventing backflow in the event of a change in temperature at the safety valve. However, these designs have disadvantages. For example, existing designs generally require a temperature difference of at least about 6° C. for actuation. This temperature difference is not always present in the event of a valve leak; therefore, risk of backflow remains in circumstances where small temperature variations exist.
Still further compounding this issue, gradual changes to temperature may occur which are not the result of (and do not signify) leakage of a safety valve. In such circumstances, a gradual rise in temperature over time, which may be attributable to other effects, may be detected by the safety valve as a possible leak, causing the safety valve to actuate and close when no leakage in fact exists. Accordingly, gas-lift safety valve designs experience the dual challenges of being required to actuate in the event of small temperature changes, while not falsely actuating in the event of gradual temperature changes.